Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
A multiple input multiple output (MIMO) scheme is used as a technique for supporting a reliable high-speed data service. The MIMO scheme uses multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Examples of the MIMO scheme include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix depending on the number of receive antennas and the number of transmit antennas can be decomposed into a plurality of independent channels. Each independent channel is referred to as a transmission layer or a stream. The number of transmission layer is referred to as a rank.
Adaptive modulation and coding (AMC) is a technique for increasing reliability of wireless communication. In the AMC, a modulation and coding scheme (MCS) is determined according to a channel state. A wireless communication system may use a channel quality indicator (CQI) for the support of the AMC. The CQI is information on a channel state between a base station (BS) and a user equipment (UE). The BS can determine the MCS used for transmission by using the CQI fed back from the UE. If it is determined that the channel state is good by using the CQI, the BS may increase a data transfer rate by increasing a modulation order or an encoding rate. Otherwise, if it is determined that the channel state is not good by using the CQI, the BS may decrease the data transfer rate by decreasing the modulation order or the encoding rate. The decrease in the data transfer rate may result in the decrease in a reception error rate.
Meanwhile, there is an ongoing standardization effort for an international mobile telecommunication-advanced (IMT-A) system aiming at the support of an Internet protocol (IP)-based multimedia seamless service by using a high-speed data transfer rate of 1 gigabits per second (Gbps) in a downlink and 500 megabits per second (Mbps) in an uplink in the international telecommunication union (ITU) as a next generation (i.e., post 3rd generation) mobile communication system. A 3rd generation partnership project (3GPP) is considering a 3GPP long term evolution-advanced (LTE-A) system as a candidate technique for the IMT-A system. It is expected that the LTE-A system is developed to further complete an LTE system while maintaining backward compatibility with the LTE system. This is because the support of compatibility between the LTE-A system and the LTE system facilitates user convenience. In addition, the compatibility between the two systems is also advantageous from the perspective of service providers since the existing equipment can be reused.
A typical wireless communication system is a single carrier system supporting one carrier. Since a data transfer rate is in proportion to a transmission bandwidth, the transmission bandwidth needs to increase to support a high-speed data transfer rate. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. For the effective use of fragmented small bands, a spectrum aggregation (or bandwidth aggregation or a carrier aggregation) technique is being developed. The spectrum aggregation technique is a technique for obtaining the same effect as when a band of a logically wide bandwidth is used by aggregating a plurality of physically non-contiguous bands in a frequency domain. By using the spectrum aggregation technique, multiple carriers can be supported in the wireless communication system. The wireless communication system supporting the multiple carriers is referred to as a multiple carrier system. The carrier may also be referred to as other terms, such as, a radio frequency (RF), a component carrier (CC), etc. The carrier will be hereinafter referred to as the CC.
Accordingly, there is a need to provide a method and apparatus for effectively transmitting a CQI in a multiple carrier system.